The present invention relates to a computed-tomography (CT) apparatus for acquiring a cross-sectional image of an object by use of X-rays, ultrasonic waves or the like, and more particularly to such a CT apparatus suitable for use as a medical diagnosis apparatus.
In recent years, CT apparatuses for acquiring cross-sectional images of objects have widely been used especially as medical diagnosis apparatuses for diagnosing patients. Such apparatuses include an X-ray CT apparatus which uses X-rays, a radiography isotope (RI) CT, ultrasonic CT and image intensifier (I.I) CT apparatuses in which the measurement of projection data is made from the circumferential directions of an object around the object to reconstruct an image, and so forth.
In a general X-ray or other type CT apparatus, a time for measuring an object at a predetermined slice position through the scan of the entire circumference of the object (or so-called 360.degree. one-slice measurement time) differs according to a plurality of measurement modes which can be changed. Usually, this measurement time is 1 to 9 seconds. Therefore, it is general that an operator of the apparatus selectively uses the optimum measurement mode in accordance with the size of an object, a part to be subjected to diagnosis or the purpose of diagnosis.
If a motion such as the motion of an object or the motion of internal organs of the object occurs during the above-mentioned measurement time, artifacts called motion artifacts are generated due to this motion. Owing to the artifacts, an accurate diagnosis from the acquired cross-sectional image becomes difficult. The well known method for solving such a technical problem is a measurement data correcting method disclosed by, for example, U.S. Pat. No. 4,580,219 issued on Apr. 1, 1986 and entitled "METHOD FOR REDUCING IMAGE ARTIFACTS DUE TO PROJECTION MEASUREMENT INCONSISTENCIES". This correction method is generally called bowel gas correction.
The bowel gas correction method will now be explained briefly by use of FIG. 6. In a usual CT apparatus, a measurement start position of a scanner is fixed beforehand at a predetermined position. The scanner starts the measurement from the predetermined measurement start position and makes a 360.degree. scan over the entire circumference of an object to obtain projection data of the object. A measurement end position assumes the same position as the measurement start position. Therefore, the first image and the last image will be consistent with each other if there is no motion of the object during an interval between the measurement start and end positions. In the actual measurement, however, when the object moves during the measurement interval, discontinuities are generated between measurement data at the measurement start and end positions. Owing to the discontinuities of measurement data, inconsistencies called misregistration differences appear between the projection data measurement start and end positions. The inconsistencies cause the generation of motion artifacts after image reconstruction. For such circumstances, data correction called bowel gas correction is made in order that the continuity of data in a predetermined rotation angle range (hereinafter referred to as correction region A) near each of the measurement start and end positions is improved even if any motion of the object is involved.
In general, the same result is given by CT projection data obtained by measuring an object from directions which are different by 180.degree.. In the bowel gas correction, the contribution of projection data to the reconstruction of a cross-sectional image is modified. More particularly, the contributions of projection data near the measurement (or scan) start position and projection data near the measurement end position providing the cause of artifacts are reduced while the contributions of projection data near a position opposite to the measurement start position with 180.degree. therebetween and projection data near a position opposite to the measurement end position with 180.degree. therebetween are increased. Namely, the cross-sectional image of one slice is generated by assigning a weight smaller than 1 to projection data in the predetermined rotation angle range (or the correction region A in FIG. 6) and assigning a weight greater than 1 to projection data in a rotation angle range (hereinafter referred to as correction region B) opposite to the correction region A. The correction regions A and B are positioned opposite to each other and have the same rotation angle range.
The weight takes the smallest value (zero) at the measurement start position and the measurement end position and is gradually increased with the increase of a distance from the measurement start position and the measurement end position. Also, the weight takes the greatest value at the middle portion of the correction region B opposite to the measurement start position and the measurement end position and is gradually decreased with the increase of a distance from the correction region B. The case where no bowel gas correction is made corresponds to the case where the same weight (1.0) is used at any position, as shown by dotted line in FIG. 6.
In a medical diagnosis CT apparatus such as X-ray CT apparatus, a measurement method is generally performed in which a contrast agent is injected into a blood vessel to emphasize constrastive differences between various tumors and normal tissues, thereby facilitating the diagnosis of a patient. In this method, the contrast agent flows away as the blood circulates. Therefore, the timing of injection of the contrasting agent and the timing of start of measurement provide important factors for accurate diagnosis. Particularly, in the tomographic imaging of a patient with an impediment in consciousness or an infant, an operator starts the measurement at a timing when the object has no motion. In such cases, the concurrency of a measurement start instruction (or operation) by the operator and the start of measurement by the diagnosis apparatus is an important task for the purpose of providing a cross-sectional image which has a high diagnostic value.
Under such backgrounds, a continuously rotatable scanner having, for example, a slip ring mounted thereon has recently been used widely. In such a scanner, the measurement start position can be set freely. Therefore, it is possible to improve the concurrency of an operator's desired measurement start timing and the start of measurement by the diagnosis apparatus, thereby shortening a time for a series of measurements.